Lately, it’s the bald eagles in a different part of the country who are making the news. Throughout December, bald eagles in Utah were dying mysteriously. The symptoms sick eagles were displaying — seizures, head tremors and paralysis — seemed to suggest West Nile virus. But the mosquitoes that spread the illness are not active so late in the year. The eagle deaths were a mystery.

Last week, Utah officials announced that they had identified the cause: It is indeed West Nile virus, but the eagles were contracting it from their consumption of dead eared grebes. Around 20,000 of the grebes have died in the Salt Lake area since November and though they may not have died from West Nile virus themselves, many are still carriers.

But the mystery isn’t solved completely. As recently as this morning, the Wildlife Rehabilitation Center of Northern Utah, where some of the sick eagles were taken for care, shared via Facebook that a number of questions still surround the deaths. They ask:

“Why so late in the year after the earliest and coldest cold snap in years?”

“Are eared grebes truly the culprit — did this species never before known to be affected by West Nile virus suddenly become vulnerable, or are they just a carrier?”

“We’ve been told that the West Nile virus is only viable in a dead body for two to three days, yet here we are 40+ days into a large grebe die-off and still receiving dead and dying eagles. Is it that the virus lasts longer in a ‘cold/frozen’ (grebe) body? Something until now, never before considered due to West Nile virus being a ‘warm weather disease?’”

The tally is now 40 dead eagles, but the loss is not expected to affect the health of the overall eagle population. Rather, it’s the usual circumstances surrounding the deaths that have wildlife experts puzzled and concerned. As Leslie McFarlane, Utah wildlife disease coordinator, tells Reuters, “This is really kind of undocumented. Eagles have been known to feed on birds infected with West Nile virus but the transmission hasn’t happened on this large of a scale. And the total number of birds we’re talking about is on a grand scale that may not have been seen before.”

Bald eagles were removed from the Endangered Species List in 2007 after a heartening return from the brink. Let’s hope this incident serves to further our knowledge of bald eagle health.

Check out these recent American Forests Global ReLeaf projects that benefit bald eagles: